Method for gettering completed gas filled lamps and the product resulting therefrom



April 29, 1969 P, M s 3,441,332

METHOD FOR GETTERING COMPLETED GAS FILLED LAMPS AND THE PRODUCTRESULTING THEREFROM Filed July 15, 1966 INVENTOR Mam/114 3014.;

BY 8 M, MIL.

ATTORNEYS United States Patent 3,441,332 METHOD FOR GETTERING COMPLETEDGAS FILLED LAMPS AND THE PRODUCT RE- SULTING THEREFROM Nickolas P.Demas, Cranford, N.J., assignor, by assignments, to Wagner ElectricCorporation, Bend, Ind., a corporation of Delaware Filed July 15, 1966,Ser. No. 565,616 Int. Cl. H01j 9/38 US. Cl. 31626 South 4 ClaimsABSTRACT OF THE DISCLOSURE The present invention relates to themanufacture of lamps and comprises an improved method for manufacturingincandescent gas filled lamps. By utilizing the method of the invention,it is possible to introduce a getter material into a sealed gas filledlamp after all other steps in the production of the lamp are completed.The resultant lamp product is of an improved quality and increasedcommercial value. The invention also includes the gettered lamp producedby this new method of manufacture.

One problem which arises during the manufacture of incandescent gasfilled lamps is the premature blackening of the lamp envelope due to achemical attack on the lamp filament by impurities in the gas fill suchas oxygen, hydrogen, carbon dioxide and water. These impurities combinewith the tungsten in the lamp filament to cause the formation oftungsten deposits on the interior of the lamp envelope. 1

One known method of preventing the undesirable tungsten deposits andalso the resulting blackening of the lamp is to introduce an alkaligetter material into an unsealed lamp at a preliminary stage in itsmanufacture. As described in my Patent No. 3,189,395, Method of MakingIncandescent Lamps, a sodium getter can be introduced into an evacuatedunsealed lamp by electrolytic conduction of sodium through the lampwall. With the application of a small voltage across the incandescentlamp filament and a molten sodium bath which is brought in contact withthe heated lamp envelope, sodium ions will pass directly through thelamp bulb. The ions become sodium atoms upon combination with electronsemitted from the lamp filament and the atoms are quickly vaporized dueto the high temperature and sub-atmospheric pressure in the envelope.The vaporized sodium solidifies on any cool portion of the envelope toform a getter spot. All impurties are absorbed by the sodium and any desired amount of neon or argon gas can be introduced into the envelopeprior to the sealing thereof.

In this regard, it has heretofore been impossible to remove a completedgas filled lamp from an assembly line after the final step in itsmanufacture and successfully introduce an absorbent alkali gettermaterial directly into the lamp by electrolytic conduction. It is nowbelieved that the electrolytic process breaks down during the getteringof a gas filled lamp and that this breakdown is due to a polarizationeflect which occurs within the lamp envelope. The polarization effectresults from the commesne' Patented Apr. 29, 1969 ice bination ofelectrons emitted by the lamp filament with ions of insert gas withinthe envelope. Since the alkali metal ions which have migrated throughthe glass are thus not neutralized, they maintain their positive chargeand prevent further migration of alkali metal ions from the supply.Discovery of this polarization effect has paved the way for the improvedmethod of gettering lamps which is the subject of the present invention.

In accordance with the method of the present invention, the abovementioned polarization effect is overcome and an alkali metal iselectrolytically passed into a completed lamp which contains a quantityof inert gas. In practicing the method of the invention, a portion ofthe gas filled envelope is heated to a temperature which is sufficientto make the glass electrically conductive but which does not soften theglass. In order to provide a supply of an alkali metal from which agetter deposit can be formed, a rod or probe is dipped into a moltencompound of the alkali metal. The quantity of alkali metal compoundwhich remains on the probe after it is withdrawn forms the supply ofalkali metal. The probe with the alkali metal compound thereon is thenbrought into contact with the heated por tion of the lamp. Electrons areemitted by the lamp filament when a source of potential is connectedacross the filament or by other means such as cold cathode emission. Theelectrons are emitted into the interior portion of the envelope whichconfines an inert gas such as argon or neon. A second source ofpotential is connected across the lamp filament and the probe carryingthe supply of alkali metal compound. The voltage of this source iscritical and must be large enough to overcome any polarization which istaking place in the lamp envelope but small enough to permit orderly andcontrolled electrolytic conduction of the alkali metal through theenvelope of the lamp. When a getter deposit of alkali metal has formedon a portion of the interior of the lamp envelope adjacent to the outerenvelope surface which is in contact with the probe carrying the supplyof alkali metal compound, the second source of potential can bedisconnected. The boundaries and location of the getter deposit aredetermined by the dimensions of the heated area of the lamp, the surfacearea of the alkali metal supply carried by the probe and the position ofthe probe as it is held in contact with the lamp envelope during theelectrolysis. The gettered lamp which is formed by practicing this novelmethod is also included in the present invention.

The present improved method of gettering completed gas filled lampsaffords several advantages over the prior art due to the fact that asealed rather than an unsealed evacuated lamp is employed. With thepresent method, the getter material can be despo-sited directly on thesealed envelope wall without the vaporization of the getter materialwhich results when unsealed lamps are used. Consequently, pressurecontrol within the lamp envelope during the gettering process is nolonger required and that step in the lamp manufacture process can beeliminated. Furthermore, by utilizing a sealed lamp envelope, theproblem of loss of getter material by vaporization through the unsealedend of the lamp is avoided. Also, when the lamp is sealed, gettermaterial cannot escape from the confines of the lamp to foul themachinery used in the process. Another advantage of the invention isthat the size and location of the getter spot can be closely controlledwithout subjecting any portion of the lamp envelope to a coolingprocess.

For a better understanding of the invention, reference may be had to theaccompanying drawings in which:

FIG. 1 is a diagram representing a stage in the process wherein aportion of the envelope of a completed gas filled lamp is heated torender it electrically conductive;

FIG. 2 is a diagram representing a succeeding stage in the processwherein alkali metal ions are passed through the heated portion of thelamp envelope by electrolytic conduction; and

FIG. 3 is a bottom view of a gettered cartridge lamp produced by themethod of the present invention.

In FIG. 1 a gas filled cartridge lamp is shown at 2 which comprises anelongated lamp envelope 4, a lamp filament 6 positioned within theenvelope and lamp leads 8 which are connected to the lamp filament 6 atconnection point 9 and extend partially outside the envelope. The lampleads 8 are sealed in position by crimping a portion 10 of the lampenvelope 4 during its manufacture, as best seen in FIG. 3. The envelopecan be filled with argon gas at a pressure of about 40 0 mm. Hg at roomtemperature prior to the sealing thereof.

It should be noted that prior to the commencement of the getteringprocess, the cartridge lamp 2 is a complete and operative lamp. Theprimary purpose of introducing a getter into the lamp is to absorb anyabnormal impurities which are present within the envelope. In additionhowever when the getter material has performed its absorbing function,it assumes a chalk-like appearance which provides visual evidence of thepurity of the atmosphere within the lamp and thereby guarantees theperformance and long life of the lamp.

During the process of the invention, the lamp 2 is mounted byconventional means (not shown) and a por tion of the lamp envelope isheated, as by a flame from a burner 14 as shown in FIG. 1. If the lampenvelope is made of lime or soda glass, a heating temperature of 300 C.will cause the envelope to become electrically conductive withoutsoftening the glass.

In FIG. 2 a probe 15 is shown as it is held in contact with the heatedportion of the lamp envelope. An end portion 16 of the probe 15 havingbeen previously dipped into a source 17 of a molten alkali metalcompound provides a supply of alkali, metal which is to be passed intothe lamp.

To eliminate the step of heating the glass envelope and the use of aprobe, a portion of the envelope can be dipped directly into the source17 of alkali metal compound. In this case, the glass is heated by themolten compound to a temperature which makes the glass electricallyconductive, but the area of sodium deposition is less easy to control.

The molten alkali metal compound is preferably a eutectic mixture ofsodium nitrate and sodium nitride. For the purpose of explanation,sodium nitrate, NaNO will be used in describing the operation of theprocess but it is to be understood that other alkali metal compounds andcombinations thereof can be employed if the same alkali metal is alsopresent in the glass Walls of the lamp.

The probe 15, or the source 17 when no probe is used, is connectedthrough a switch 18 to the positive terminal of a source of potential20, the negative terminal of which is connected to the lamp lead 8. Asecond source of potential 22 is placed "across the lamp leads 8 and thefilament 6 to thereby heat the filament to incandescenee and initiatethe emission of electrons. Since molten NaNO disassociates into Na+ andNO;, ions, the positive sodium ions migrate into and through theelectrically conductive glass of the lamp envelope 4 under the influenceof the negative potential on the filament 6 due to source 20. Electronsemitted by the filament are attracted in the direction of the sodiumsupply by virtue of the positive potential of the source 20. At thisstage in the process, the voltage of the source becomes critical if theprocess of electrolysis is to continue with the neutralization of thesodium ions and the formation of a getter spot. Since the lamp is gasfilled, an excessive number of the electrons from the filament cancombine with gas ions if improper voltages are employed. As a result ofthis combination, positive sodium ions on the interior of the envelopeare not neutralized and additional sodium ions will fail to enter thelamp by virtue of the polarization effect at the inner wall thereof. Theminimum value of the voltage of source 20 which overcomes polarizationis 300 volts. Voltages below 300 are insufficient to avoid polarizationby the migrated alkali ions. The variation of potential above theminimum voltage value is related to the type of lamp being gettered, thetemperature of the filament, the pressure inside the lamp, the type ofinert gas in the lamp and the distance between the filament and the lampwall. As an example, using a cartridge lamp filled with argon to about400 mm. Hg (at room temperature), a spacing of 4 mm. between thefilament and lamp wall, a filament temperature of 2300 C., and apotential of 400 volts DC. for source 20, there is a consistentneutralization of sodium to form the getter spot 24.

In FIG. 3 a cartridge lamp is shown with the greater spot 24 located onan inner portion of the envelope 4. The position of the spot 24 withregard to the remainder of the lamp envelope is precisely determined bythe position of the probe 15 carrying the supply of the molten alkalicompound. In addition, the area of the spot is directly related to thearea of the supply of the molten compound carried by the probe 15 whichis held in contact with the envelope during the process. The locationand size of the spot is of particular importance in the gettering of acartridge lamp where the getter spot must be small to permit fullillumination from the lamp. The invention however is not limited to thegetten'ng of cartridge type lamps.

I claim:

1. A method of providing a getter for a gas filled lamp, comprising thesteps of:

(a) heating a portion of the selected glass envelope of a completed gasfilled lamp to a temperature sufficient to cause the glass to becomeelectrically con ductive;

(b) placing the heated portion of the sealed lamp in contact with asupply of a molten alkali metal compound;

(c) initiating electron emission from the lamp filament into theenvelope which is occupied by the inert gas;

(d) applying a potential across the lamp filament and the supply ofmolten alkali metal compound, said potential providing a sufiicientvoltage to bring about electrolytic conduction of the alkali metalthrough the heated portion of the glass envelope and to overcomepolarization within the envelope; and

(e) discontinuing the application of said potential when a getterdeposit of alkali metal is formed on a portion of the interior of theenvelope adjacent the heated envelope surface which is in contact withthe molten compound, said alkali metal being permanently retained withinthe glass envelope.

2. The method of providing a getter for a gas filled lamp according toclaim 1 wherein the molten alkali metal compound is a eutectic mixtureof sodium nitrate and sodium nitride and said getter deposit is sodium.

3. The method of providing a getter for a gas filled lamp according toclaim 1 wherein said potential is at least 300 volts.

4. The method as in claim 1 wherein the lamp in which the getter depositis formed is a cartridge type lamp.

References Cited UNITED STATES PATENTS 3,183,051 5/1965 Schmidt 3l633,189,395 6/1965 Demas 313-222 X JAMES W. LAWRENCE, Primary Examiner.DAVID OREILLY, Assistant Examiner.

U.S. Cl. X.R. 313-174, 179, 181

Attesting Officer U.S. DEPARTMENT OF COMMERCE PATENT OFFICE Washington,0.6. 20231 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 3,441,332 April 29, 1969 Nickolas P. Demas It is certified thaterror appears in the above identified patent and that said LettersPatent are hereby corrected as shown below:

Column 4, line 32, "selected should read sealed Signed and sealed this14th day of April 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Commissioner of Patents Edward M. Fletcher, Jr.

